1. Technical Field
This technology pertains generally to magnetic random access memory (MRAM), and more particularly to multiple bit-per-cell magnetoelectric random access memory (MBPC-MeRAM).
2. Background Discussion
Modern electronic devices increasingly incorporate significant amounts of solid state memory. The electronics industry continually seeks higher density devices that provide low power consumption. Magnetic memory devices by their nature provide non-volatile characteristics, and are drawing increasing attention as a next generation memory type.
Magnetic Random Access Memory (MRAM) has generally been implemented with a magnetic-field-controlled or electric-current-controlled write mechanism. Data within an MRAM is not stored as electric charge or current flows, as in a conventional RAM, but instead by magnetic storage elements formed from ferromagnetic plates, separated by insulating material. In a magnetoresistive RAM, reading is performed in response to measuring electrical resistance of the cell, which changes due to the orientation of the fields in the two plates. Data writes are performed by inducing a magnetic field in response to current through write lines to change magnetic orientation.
More recently, spin transfer torque (STT) techniques used spin-aligned (“polarized”) electrons to directly torque the domains. In particular, if the electrons flowing into a layer are forced to change their spin, this will develop a torque that will be transferred to the nearby layer. Using STT, power requirements are substantially lowered. There has been a significant amount of research and development on STT-RAM, where electric currents are driven through a magnetic tunnel junction (MTJ) bit to switch it and thus to write information into it. The use of currents for writing STT-RAM, however, still involves substantial energy dissipation, and provides a limited maximum memory array density since each magnetic bit requires a large access transistor to drive its large write current, which also limits its scalability.